System and method for transmitting data in a network

ABSTRACT

A system and method for transmitting data in a network comprising the steps of determining a traffic congestion variable of a data transmission node arranged to receive data from one or more source nodes of the network, using the traffic congestion variable to select a preferred transmission mode for use by the one or more source nodes to transmit data to the data transmission node, and switching an operating transmission mode of each of the one or more source nodes to the preferred transmission mode such that the one of more source nodes transmit data to the data transmission node with the preferred transmission mode.

TECHNICAL FIELD

The present invention relates to a system and method for transmittingdata in a network and particularly, although not exclusively, to a dualmode data transmission system and method for transmitting data inoptical networks.

BACKGROUND

With an ever increasing data traffic in data transmission networks suchas the Internet, efficient and reliable data transmission is highlydesirable for handling and communicating a large amount of data. Todate, different data transmission methods have their own advantages andlimitations. For example, some data transmission methods may be fasterin speed whilst some other data transmission methods may be lesssusceptible to data transmission loss.

Choosing which method to use in the right instances would be helpful inestablishing an efficient communication transmission network. This, inturn, may improve the efficiency of data transmission in the network andhence provides additional flexibility and enhanced cost-effectiveness tothe data transmission system.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided a method for transmitting data in a network comprising thesteps of determining a traffic congestion variable of a datatransmission node arranged to receive data from one or more source nodesof the network; using the traffic congestion variable to select apreferred transmission mode for use by the one or more source nodes totransmit data to the data transmission node; and switching an operatingtransmission mode of each of the one or more source nodes to thepreferred transmission mode such that the one of more source nodestransmit data to the data transmission node with the preferredtransmission mode.

In an embodiment of the first aspect, the operating transmission mode isswitched to the preferred transmission mode when the preferredtransmission mode is different to the operating transmission mode.

In an embodiment of the first aspect, the preferred transmission mode isselected by comparing the traffic congestion variable with at least onepredetermined threshold.

In an embodiment of the first aspect, the preferred transmission mode isselected based on the traffic congestion variable being above or belowthe at least one predetermined threshold.

In an embodiment of the first aspect, the operating transmission modeand the preferred transmission mode includes an optical burst switching(OBS) mode or an optical circuit switching (OCS) mode.

In an embodiment of the first aspect, the at least one predeterminedthreshold comprises a congestion threshold and a recovery threshold.

In an embodiment of the first aspect, the OCS mode is selected for useby the one or more source nodes when the traffic congestion variable isabove the congestion threshold.

In an embodiment of the first aspect, the OBS mode is selected for useby the one or more source nodes when the traffic congestion variable isbelow the recovery threshold.

In an embodiment of the first aspect, the congestion threshold is largerthan or equal to the recovery threshold.

In an embodiment of the first aspect, the congestion threshold and therecovery threshold are predetermined.

In an embodiment of the first aspect, the data transmission node isarranged to transmit data in both optical burst switching (OBS) mode andoptical circuit switching (OCS) mode.

In an embodiment of the first aspect, the method for transmitting datain a network further comprises a step of communicating a switchingsignal to the one or more source nodes to switch the one or more sourcenodes to the preferred transmission mode.

In an embodiment of the first aspect, each source node is arranged toswitch its operating transmission mode based on the traffic congestionvariables of one or more data transmission nodes.

In accordance with a second aspect of the present invention, there isprovided a system for transmitting data in a network comprising acontroller arranged to determine a traffic congestion variable of a datatransmission node arranged to receive data from one or more source nodesof the network; a selection module arranged to select a preferredtransmission mode based on the traffic congestion variable for use bythe one or more source nodes to transmit data to the data transmissionnode; and a switching module arranged to switch an operatingtransmission mode of each of the one or more source nodes to thepreferred transmission mode such that the one of more source nodestransmit data to the data transmission node with the preferredtransmission mode.

In an embodiment of the second aspect, the switching module switches theoperating transmission mode to the preferred transmission mode when thepreferred transmission mode is different to the operating transmissionmode.

In an embodiment of the second aspect, the selection module selects thepreferred transmission mode by comparing the traffic congestion variablewith at least one predetermined threshold.

In an embodiment of the second aspect, the selection module selects thepreferred transmission mode based on the traffic congestion variablebeing above or below the at least one predetermined threshold.

In an embodiment of the second aspect, the operating transmission modeand the preferred transmission mode includes an optical burst switching(OBS) mode or an optical circuit switching (OCS) mode.

In an embodiment of the second aspect, the at least one predeterminedthreshold comprises a congestion threshold and a recovery threshold.

In an embodiment of the second aspect, the selection module selects theOCS mode for use by the one or more source nodes when the trafficcongestion variable is above the congestion threshold.

In an embodiment of the second aspect, the selection module selects theOBS mode for use by the one or more source nodes when the trafficcongestion variable is below the recovery threshold.

In an embodiment of the second aspect, the congestion threshold islarger than or equal to the recovery threshold.

In an embodiment of the second aspect, the congestion threshold and therecovery threshold are predetermined.

In an embodiment of the second aspect, the data transmission node isarranged to transmit data in both optical burst switching (OBS) mode andoptical circuit switching (OCS) mode.

In an embodiment of the second aspect, the system for transmitting datafurther comprises a communication module arranged to communicate aswitching signal to the one or more source nodes to switch the one ormore source nodes to the preferred transmission mode.

In an embodiment of the second aspect, the switching module is arrangedto switch the operating transmission mode of each source node based onthe traffic congestion variables of one or more data transmission nodes.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 shows an exemplary 5-node network arranged to use a datatransmission method for transmitting data in one embodiment of thepresent invention;

FIG. 2 shows the data transmission method used by the network for FIG.1;

FIG. 3 shows an exemplary 13-node NSFNet network topology arranged touse the data transmission method of FIG. 2; and,

FIG. 4 is a graph showing the network throughput for the network of FIG.3 with 50 channels per trunk at different data transmission modes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The applicant has devised, through experimentation and trials, thatoptical burst switching (OBS) is an optical networking technology thatfacilitates one-way dynamic resource (e.g. wavelength channels)reservation of data flows suited to all-optical networks whereas opticalcircuit switching (OCS) and optical flow switching (OFS) are opticalnetworking technologies in which end-to-end network resources arereserved in advance so that the data transmitted in the network willalways be able to reach their destinations.

In OBS networks, data that are to be transmitted to the same destinationare aggregated at ingress (source) nodes to form bursts. A burst mayinclude data from various sources (when OBS operates at the corenetwork) or from a single source (when OBS operates end-to-end). Controlpacket or signal is sent ahead of a burst to reserve wavelength channelsalong the burst transmission path in a hop by hop manner.

Since the wavelength channels are reserved hop by hop in OBS, theresource reservation time required prior to initiating data transmissionis generally shorter than in end-to-end channel reservation schemes suchas OCS. Preferably, OBS can be used to provide bandwidth on demand whenfast setup time is needed. Another benefit of OBS over OCS is that inOBS light-paths are fully utilized during burst transmission whereas inOCS these light-paths may not be fully utilized.

In OBS, however, a burst of data, after utilizing certain networkresources, may be dumped as a result of congestion in the network andnever reaches the destination. The utilization of resources (e.g.wavelength channels) by the bursts that will eventually be blocked ordumped during the transmission process represents utilized networkresources that do not lead to network throughput.

This has a detrimental effect on the network as these wasted resourcescan otherwise be used by other bursts. In OCS networks, the channels areused by the traffic or bursts that will certainly reach the destinationand therefore no utilized resources are wasted. The problem in whichsome resources utilized by the bursts will eventually be dumped andfailed to reach the destination is a peculiar weakness of OBS networks.Furthermore, an overloaded or congested trunk in a network will causeproblems to other neighbouring trunks. This may in turn cause the othertrunks to become congested and hence the reliability and efficiency ofdata transmission will be compromised.

Referring now to FIG. 1, there is shown an exemplary 5-node network 100arranged to use a data transmission method for transmitting data in oneembodiment of the present invention. The method comprising the steps ofdetermining a traffic congestion variable of a data transmission nodearranged to receive data from one or more source nodes of the network;using the traffic congestion variable to select a preferred transmissionmode for use by the one or more source nodes to transmit data to thedata transmission node; and switching an operating transmission mode ofeach of the one or more source nodes to the preferred transmission modesuch that the one of more source nodes transmit data to the datatransmission node with the preferred transmission mode.

As shown in FIG. 1, the exemplary network 100 comprises two source nodes102, two data transmission nodes 104 and a destination node 106. Thesenodes are connected to one another through trunks 108 having a pluralityof channels 110. During operation of the network, data is transmittedfrom the source nodes 102 to the destination nodes 106 through theintermediate data transmission nodes 104. In some embodiments, the datatransmission nodes 104 may be arranged to transmit or receive data fromdifferent source nodes 102 at the same time. In other words, the trunk108 of the data transmission nodes 104, at any particular time, may betransmitting data that originates from different source nodes 102.Preferably, all the nodes in the network 100 are arranged and adapted totransfer data in a plurality of data transmission modes. These datatransmission modes may include optical circuit switching (OCS), opticalburst switching (OBS), optical flow switching (OFS) or any other opticalor electrical data transmission modes.

In a preferred embodiment, the nodes in the network as shown in FIG. 1are arranged to transmit data using both OBS and OCS modes so as toprovide an optimal data transmission performance. Preferably, thenetwork 100 of FIG. 1 has a network congestion control that switches thedata transmission modes at the source nodes 102 depending on the trafficin the data transmission nodes. In a preferred embodiment, the sourcenodes 102 use OCS mode to transmit data when the traffic in the network100 or the data transmission nodes 104 is congested. OBS mode is usedwhen the traffic in the network 100 or the data transmission nodes 104is low.

In one embodiment, the data transmission node 104 is arranged todetermine a congestion variable relating to its own traffic condition.The traffic congestion variable may be related to the channel 110utilization of the data transmission node 104, the processing timerequired at the data transmission node 104, etc. The determination ofthe variable may be performed by a controller associated with the datatransmission nodes 104. In other embodiments, the controller may beexternal or the determination of the variable may be performed by othermodules or units associated with the nodes.

The determined traffic congestion variable is then compared with atleast one threshold value to determine if data transmission modeswitching is required at the source nodes 102. This step can beperformed by the controller at the data transmission nodes 104 or aseparate controller located externally or in other nodes. In oneembodiment, a preferred transmission mode will be selected based on thetraffic congestion variable by a selection module. In a preferredembodiment, the threshold values comprise a congestion thresholdindicating that the channels 110 are congested and a recovery thresholdindicating that the channels 110 are not fully utilized.

Preferably, the value of the congestion threshold is larger than orequal to that of the recovery threshold. Also, the thresholds arepreferably predetermined. In some other embodiments, there may be anynumber of thresholds or the thresholds may be dynamically adjusted. Theuse of two thresholds is advantageous over the use of a single thresholdin that it prevents excessive and unnecessary rapid switching oftransmission mode in the source nodes 102 due to periodic small trafficfluctuation about the threshold value. This prevents inducingunnecessary damages to the switching modules or the nodes.

In an embodiment, if it is determined that the traffic congestionvariable exceeds the congestion threshold, a switching signal will becommunicated to the source nodes 102 that use the trunk 108 of the datatransmission node 104 to switch the data transmission mode of thesesource nodes 102 to OCS. On the other hand, if it is determined that thetraffic congestion variable falls below the recovery threshold, aswitching signal will be communicated to the source nodes 102 that usethe trunk 108 of the data transmission node to switch the datatransmission mode of these source nodes 102 to OBS. Preferably, thecommunication of the switching signal to the source nodes 102 isperformed by a communication module associated with the system or thenodes.

The source nodes 102, upon receiving the switching signal, will switchto the preferred transmission mode. In these cases, the switching may beperformed by a switching module associated with the source nodes 102. Insome embodiments, the source nodes 102 may be arranged to receiveswitching signals from more than one transmission nodes 104. Preferably,the data transmission mode of the source nodes 102 will be switched ifthe present operating transmission mode is different to the datatransmission mode indicated by the switching signal. The data that aresent from the source nodes 102 after the source nodes receive theswitching signal may also be transmitted using the preferredtransmission mode. The transmission mode of the data that are alreadybeing transmitted in the network before the source nodes 102 receive theswitching signal will be substantially unaffected.

In operation, when the utilization of the trunk 108 of a transmissionnode 104 exceeds the congestion threshold, a switching signal will besent to all the source nodes 102 that use the trunk and turn thetransmission mode of these source nodes into OCS. The bursts that aresent from these source nodes 102 after the source nodes receive theswitching signal will be transmitted in OCS mode and control packetswill be sent end-to-end to reserve channels 110 for the bursts. Thebursts that are already being transmitted in the nodes before the sourcenodes 102 receive the switching signal will remain to be transmitted hopby hop using OBS mode. Preferably, the bursts that are being sent by theother source nodes will not be affected. This embodiment is particularlyadvantageous as the two data transmission modes, OBS and OCS, will existin the network at the same time.

As mentioned earlier, The bursts that are transmitted in OCS mode willnot occupy the channels 110 until an end-to-end channel reservation isestablished. This prevents the bursts that will eventually be dumpedduring the transmission process from using the channels 110 and in turnreduces the load for the congested trunk 108. As a result, theutilization of the congested trunks will be gradually decreased untilthe traffic congestion variable falls below the recovery threshold. Thisimplies that the network or the transmission nodes are no longercongested. A switching signal may then be sent to all the source nodes102 that use the trunks 108 to turn the transmission mode in thesesource nodes back into OBS. In this way, the network will have animproved transmission speed or enhanced reliability when compared tousing either one of OBS or OCS for data transmission.

Although a 5-node network 100 using the data transmission method in anembodiment of the present invention is shown in FIG. 1, it should beunderstood that the method as illustrated can be implemented on othernetworks having any number of nodes or of different node arrangements.Also, the number and position of the source nodes, data transmissionnodes and destination nodes can be freely chosen and arranged.

FIG. 2 shows the method 200 for transmitting data used by the network ofFIG. 1. In this method 200, firstly, a traffic congestion variableindicating the utilization of a data transmission node is determined202. Preferably, each data transmission node has its own trafficcongestion variable. The traffic congestion variable determined is thenused to select a preferred transmission mode from OBS and OCS modes 204.Particularly, the selection of the preferred transmission mode is basedon comparing the traffic congestion variable with a congestion thresholdand a recovery threshold. OCS mode is selected when the variable exceedsthe congestion threshold and OBS mode is selected when the variablefalls below the recovery threshold.

Upon determining or selecting the preferred transmission mode, aswitching signal indicating the preferred transmission mode may begenerated and communicated to the source nodes that are utilizing thedata transmission node 206. In some embodiments, the switching signalmay be generated at the source nodes and thus no communication of theswitching signal to the source nodes are required. The switching signalcommands the source nodes to switch their data transmission mode to thepreferred transmission mode. In response to the switching signal, thedata transmission mode of these source nodes is switched to thepreferred data transmission mode when the operating transmission mode ofthese source nodes are different to the preferred transmission mode 208.

Turning now to FIG. 3, an exemplary 13-node NSFNet network topology 300as shown is used to illustrate the performance of the dual mode datatransmission method 200 of FIG. 2. By choosing all possibleorigin-destination pairs with shortest path routing where a tie isbroken randomly, a graph 400 showing the network throughput for thenetwork 300 of FIG. 3 with 50 channels per trunk at different datatransmission modes is shown in FIG. 4. In FIG. 4, performance ofdifferent sets of congestion and recovery thresholds used in the dualmode model are compared.

As shown in FIG. 4, when the network 300 is under-loaded and theprobability of blocking (data loss) is negligible, the networkthroughput is about the same for OBS and OCS. As the offered load(traffic) increases, blocking probability increases in OBS. This leadsto congestion collapse of network throughput. On the other hand, in OCSwhere no resources are used for the traffic that will eventually be lostor dumped during the transmission process, the network resources areutilized more efficiently albeit the data transmission speed may beslower.

In an embodiment of the data transmission method of the presentinvention, the bursts are transmitted in OBS mode when the utilizationof the nodes or network 300 is smaller than the recovery threshold andOCS mode when the utilization of the nodes or network 300 exceeds thecongestion threshold. As such, the throughput of the dual modetransmission network 300 is expected to be close to that of OCS whereasthe data transmission speed is expected to improve when compared tousing only OCS mode.

As shown in FIG. 4, when the congestion and recovery thresholds are bothset to 90%, the network throughput is comparable to that of using OCS.On the other hand, when the congestion and recovery thresholds are bothset to 100%, the network throughput is between that of OBS and OCS. Thiscan be explained by that more bursts are being transmitted in OCS modewhen the thresholds are at 90% than when they are at 100%. The networkthroughput of that when the thresholds are both set to 90% is thereforecloser to the performance of OCS.

It should be noted, however, that how close the network throughput inthe preferred embodiment of the dual mode transmission illustrated inthe present invention to that of OCS is dependent on the proportion ofbursts being transmitted in OCS mode, which in turn depends on thesettings of the recovery and congestion thresholds. Particularly, themore the proportion of bursts is transmitted in OCS mode, the closer thenetwork throughput will be to that of OCS.

Tables 1, 2 and 3 below show the effect of choosing different values forthe recovery and congestion thresholds on network throughput and on theproportions of bursts that are transmitted in OBS and OCS modes for theexemplary network 300.

TABLE 1 Network throughput (Erlangs) for different thresholds Networkthroughput (Erlangs) Offered load(Erlangs) 1 3 5 8 10 12 15 A. recovery155.9294 399.9649 535.5494 669.8019 726.6679 768.6698 812.8134 threshold= 70 congestion threshold = 90 B. recovery 155.9267 378.8076 524.2964659.8142 718.1768 763.7998 810.2687 threshold = 90 congestion threshold= 90 C. recovery 155.9765 377.4525 522.0722 656.7204 715.8017 760.4236808.9138 threshold = 90 congestion threshold = 95

TABLE 2 Proportions of bursts that are transmitted in OBS modeProportions of bursts that are transmitted in OBS mode Offeredload(Erlangs) 1 3 5 8 10 12 15 A. recovery 0.9960 0.3806 0.2010 0.05860.0239 0.0123 0.0052 threshold = 70 congestion threshold = 90 B.recovery 0.9979 0.5448 0.3173 0.1532 0.0940 0.0598 0.0325 threshold = 90congestion threshold = 90 C. recovery 0.9996 0.5823 0.3580 0.1917 0.12580.0847 0.0492 threshold = 90 congestion threshold = 95

TABLE 3 Proportions of bursts that are transmitted in OCS modeProportions of bursts that are transmitted in OCS mode Offeredload(Erlangs) 1 3 5 8 10 12 15 A. recovery 0.0040 0.6194 0.7990 0.94140.9761 0.9877 0.9948 threshold = 70 congestion threshold = 90 B.recovery 0.0021 0.4552 0.6827 0.8468 0.9060 0.9402 0.9675 threshold = 90congestion threshold = 90 C. recovery 0.0004 0.4177 0.6420 0.8083 0.87420.9153 0.9508 threshold = 90 congestion threshold = 95

As shown in Tables 1, 2 and 3, when the offered load is 1 erlang, thenetwork 300 is under-loaded and the throughput of OBS and OCS are almostthe same. Therefore, the throughputs of the dual mode network 300 withdifferent threshold settings do not have much difference. When theoffered load is equal to or more than 3 erlangs, the network 300 becomescongested and about half of the total number of bursts is transmitted inOCS mode (as shown in Tables 2 and 3). This leads to a more obviousdifference in network throughputs. As setting A (recovery threshold: 70& congestion threshold: 90) uses OCS mode more frequently than setting B(recovery threshold: 90 & congestion threshold: 90) than setting C(recovery threshold: 90 & congestion threshold: 95), more bursts arebeing transmitted in OCS mode in setting A than setting B than settingC. Therefore, the network throughput is larger for setting A thansetting B than setting C. Tables 1, 2 and 3 illustrate that the designof the thresholds is a trade-off between speed and reliability (lessdata loss). For a higher reliability, the bursts should be transmittedmore in OCS mode. For faster data transmission, the bursts should betransmitted more in OBS mode.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

Any reference to prior art contained herein is not to be taken as anadmission that the information is common general knowledge, unlessotherwise indicated.

1. A method for transmitting data in a network comprising the steps of:determining a traffic congestion variable of a data transmission nodearranged to receive data from one or more source nodes of the network;using the traffic congestion variable to select a preferred transmissionmode for use by the one or more source nodes to transmit data to thedata transmission node; and switching an operating transmission mode ofeach of the one or more source nodes to the preferred transmission modesuch that the one of more source nodes transmit data to the datatransmission node with the preferred transmission mode.
 2. A method fortransmitting data in accordance with claim 1, wherein the operatingtransmission mode is switched to the preferred transmission mode whenthe preferred transmission mode is different to the operatingtransmission mode.
 3. A method for transmitting data in accordance withclaim 2, wherein the preferred transmission mode is selected bycomparing the traffic congestion variable with at least onepredetermined threshold.
 4. A method for transmitting data in accordancewith claim 3, wherein the preferred transmission mode is selected basedon the traffic congestion variable being above or below the at least onepredetermined threshold.
 5. A method for transmitting data in accordancewith claim 4, wherein the operating transmission mode and the preferredtransmission mode includes an optical burst switching (OBS) mode or anoptical circuit switching (OCS) mode.
 6. A method for transmitting datain accordance with claim 5, wherein the at least one predeterminedthreshold comprises a congestion threshold and a recovery threshold. 7.A method for transmitting data in accordance with claim 6, wherein theOCS mode is selected for use by the one or more source nodes when thetraffic congestion variable is above the congestion threshold.
 8. Amethod for transmitting data in accordance with claim 6, wherein the OBSmode is selected for use by the one or more source nodes when thetraffic congestion variable is below the recovery threshold.
 9. A methodfor transmitting data in accordance with claim 6, wherein the congestionthreshold is larger than or equal to the recovery threshold.
 10. Amethod for transmitting data in accordance with claim 6, wherein thecongestion threshold and the recovery threshold are predetermined.
 11. Amethod for transmitting data in accordance with claim 5, wherein thedata transmission node is arranged to transmit data in both opticalburst switching (OBS) mode and optical circuit switching (OCS) mode. 12.A method for transmitting data in accordance with claim 1, furthercomprising a step of: communicating a switching signal to the one ormore source nodes to switch the one or more source nodes to thepreferred transmission mode.
 13. A method for transmitting data inaccordance with claim 1, wherein each source node is arranged to switchits operating transmission mode based on the traffic congestionvariables of one or more data transmission nodes.
 14. A system fortransmitting data in a network comprising: a controller arranged todetermine a traffic congestion variable of a data transmission nodearranged to receive data from one or more source nodes of the network; aselection module arranged to select a preferred transmission mode basedon the traffic congestion variable for use by the one or more sourcenodes to transmit data to the data transmission node; and a switchingmodule arranged to switch an operating transmission mode of each of theone or more source nodes to the preferred transmission mode such thatthe one of more source nodes transmit data to the data transmission nodewith the preferred transmission mode.
 15. A system for transmitting datain accordance with claim 14, wherein the switching module switches theoperating transmission mode to the preferred transmission mode when thepreferred transmission mode is different to the operating transmissionmode.
 16. A system for transmitting data in accordance with claim 15,wherein the selection module selects the preferred transmission mode bycomparing the traffic congestion variable with at least onepredetermined threshold.
 17. A system for transmitting data inaccordance with claim 16, wherein the selection module selects thepreferred transmission mode based on the traffic congestion variablebeing above or below the at least one predetermined threshold.
 18. Asystem for transmitting data in accordance with claim 17, wherein theoperating transmission mode and the preferred transmission mode includesan optical burst switching (OBS) mode or an optical circuit switching(OCS) mode.
 19. A system for transmitting data in accordance with claim18, wherein the at least one predetermined threshold comprises acongestion threshold and a recovery threshold.
 20. A system fortransmitting data in accordance with claim 19, wherein the selectionmodule selects the OCS mode for use by the one or more source nodes whenthe traffic congestion variable is above the congestion threshold.
 21. Asystem for transmitting data in accordance with claim 19, wherein theselection module selects the OBS mode for use by the one or more sourcenodes when the traffic congestion variable is below the recoverythreshold.
 22. A system for transmitting data in accordance with claim19, wherein the congestion threshold is larger than or equal to therecovery threshold.
 23. A system for transmitting data in accordancewith claim 19, wherein the congestion threshold and the recoverythreshold are predetermined.
 24. A system for transmitting data inaccordance with claim 18, wherein the data transmission node is arrangedto transmit data in both optical burst switching (OBS) mode and opticalcircuit switching (OCS) mode.
 25. A system for transmitting data inaccordance with claim 14, the system further comprising a communicationmodule arranged to communicate a switching signal to the one or moresource nodes to switch the one or more source nodes to the preferredtransmission mode.
 26. A system for transmitting data in accordance withclaim 14, wherein the switching module is arranged to switch theoperating transmission mode of each source node based on the trafficcongestion variables of one or more data transmission nodes.